This invention relates to alpha-olefin polymerization catalysts, and more particularly, to catalysts useful in the preparation of polyalpha-olefins, and especially polyethylene, of broad molecular weight distributions consisting essentially of (A) an organometallic promoter; and (B) the reaction product of an alkylaluminum halide with at least one titanium(IV) compound and at least one zirconium(IV) compound.
In the polymerization of alpha-olefins it is desirable to employ catalysts which allow for the production of polymeric products having broad molecular weight distributions because such polymers are easily processed.
One approach in attempting to provide such catalysts has been to employ mixtures of compounds of different transition metals. For example, U.S. Pat. No. 3,678,025, to Birrell, discloses mixed catalyst systems comprising (1) the reaction product of a Group IA, IIA, or IIIA metal alkyl, alkylhalide, or alkylhydride reducing agent and a halide, oxyhalide, alcoholate, alkoxide, or ester of a Group IVB, VB, or VIB metal other than zirconium; and (2) the reaction product of a Group IA, IIA, or IIIA metal alkyl, alkylhalide, or alkylhydride reducing agent and a zirconium halide or oxyhalide. The disclosed catalysts are useful in the preparation of polyalpha-olefins having relatively broad molecular weight distributions, however, catalyst activities are relatively low. Furthermore, the disclosed catalysts are disadvantageous because two separate preparations are required.
U.S. Pat. No. 3,899,477, to Altamore et al. also discloses catalysts for the polymerization of alpha-olefins to polymers of broad molecular weight distributions. The disclosed catalysts comprise (1) a mixture of an alkylaluminum sesquialkoxide with a trialkylaluminum; and (2) a component prepared by reacting a titanium halide, a vanadium halide, and an alkylaluminum compound in amounts such that the ratio of aluminum to the sum of titanium and vanadium ranges from 0.1 to 0.5. While such catalysts exhibit relatively high activities and allow for preparation of polyalpha-olefins having relatively broad molecular weight distributions, the catalysts are disadvantageous because a mixture of an alkylaluminum sesquialkoxide and a trialkylaluminum compound must be used as the promoter to achieve broad polymer molecular weight distributions. The preparation and use of this mixed promoter can add cost and inconvenience to polymerization processes.
From the foregoing, it is apparent that an improved catalyst for the polymerization of alpha-olefins to polymers of broad molecular weight distributions would be desirable. It is an object of this invention to provide such an improved catalyst and a method for the preparation thereof. Another object of the invention is to provide for the polymerization of alpha-olefins in the presence of such catalysts. Other objects of the invention will be apparent to persons of skill in the art from the following description and the appended claims.
We have found that the objects of this invention can be achieved by reacting an alkylaluminum component, at least one titanium(IV) halide, alkoxide, or alkoxyhalide, and at least one zirconium(IV) halide, alkoxide, or alkoxyhalide to form a solid catalyst component which, when combined with an organometallic promoter, catalyzes the polymerization of alpha-olefins to polymers of broad molecular weight distributions in sufficiently high yields, relative to the amount of catalyst employed, that separation of catalyst residues from the polymeric products is unnecessary. Although the mechanism by which the solid component is formed is not known, it is believed that the preparative reaction involves a reduction of the titanium compound and the formation of solid, lower valent titanium halides which bond to or are coated onto solid zirconium compounds. Accordingly it is believed that the composition of the invented catalyst components differs substantially from the mixture of separately prepared titanium and zirconium components disclosed in the Birrell patent. Additionally, unlike the catalysts of that patent, the invented catalyst components require only one preparation and are generally superior in activity. The invented catalyst components are clearly distinguishable from those of the Altamore et al. patent because the former are prepared from titanium and zirconium compounds, with vanadium compounds such as are employed according to Altamore et al. being excluded. Additionally, the invented catalyst components allow for the attainment of broad polymer molecular weight distributions without the need for a mixed promoter. The invented catalysts also exhibit a relatively high sensitivity to hydrogen which is commonly employed in alpha-olefin polymerization to regulate polymer molecular weights. Accordingly, the invented catalysts can be employed to prepare polymers having a wide range of flow properties. Another beneficial feature of the invented catalyst components is that variations in the breadth of polyalpha-olefin molecular weight distributions can be achieved by simply varying the relative amounts of titanium and zirconium compounds employed in preparation of the catalyst components.
Other catalysts based on combinations of titanium and zirconium have been disclosed and may be of interest to the present invention. U.S. Pat. No. 2,930,785, to Edmonds, Jr., discloses catalysts comprising (1) a compound of a Group IVB metal, including titanium(IV) and zirconium(IV) alkoxides, aryloxides, alkenyloxides, or acyloxides, (2) a halide of silicon, germanium, tin, lead, phosphorus, arsenic, antimony, or bismuth, and (3) an elemental metal having an oxidation potential higher than that of the Group IVB metal contained in (1). It also is disclosed that mixtures of compounds of the Group IVB metals can be employed as (1), however, there is no suggestion to prepare a catalyst component by reacting such a mixture with an alkylaluminum compound, nor does Edmonds, Jr. suggest the use of an organometallic promoter. The invented catalysts are distinguishable from those of Edmonds, Jr. for the additional reason that the former do not include the metal halide and elemental metal components disclosed in the patent. Additionally, Edmonds, Jr. is silent with respect to polymer molecular weight distributions and catalyst activities are insufficient to eliminate the need for removal of catalyst residues from polymeric products produced in the presence of the catalysts. U.S. Pat. No. 3,069,364, to D'Alelio, discloses alpha-olefin polymerization catalysts prepared by grinding zirconium metal with titanium dichloride for at least about 48 hours at ambient temperature. This patent also is silent with respect to polymer molecular weight distributions. West German Pat. No. 1,176,868, to Farbenfabriken Bayer A.G., discloses catalysts for the polymerization of ethylene to polymers of controlled molecular weights comprising zirconium dihalides or mixtures thereof with other metal halides, including those of divalent titanium. It also is disclosed that polymer molecular weights vary depending on the choice of metal halide which is mixed with the zirconium dihalide.